Walter Mondale. 'I wasn't ready in '74,' he says. 'Now I am.'

It is a steamy Missouri evening, steeped in the kind of heat only fireflies and politicians flit around in by choice. But hundreds of Democratic Party faithful have turned out to wilt in their Saturday-night party clothes for a glimpse of candidate Walter Mondale, the man who would be president.

As they funnel into the vast Hearnes Multi-Purpose Building on the University of Missouri campus, they are greeted by double blasts: heavy air conditioning and a band playing ''When Irish Eyes Are Smiling.''

It looks like a scene from ''The Last Hurrah'': farewell to a legendary Irish pol named James Kirkpatrick, the retiring Missouri secretary of state. The air is full of smoke, banter, and the scent of green carnations decorating long tables, arranged like a church supper.

The Democrats are just dishing into fresh strawberry appetizers when the spotlight hits the door. The band strikes up ''Happy Days Are Here Again,'' and all the speakers march in. Former Vice-President Mondale, toward the end, moves slowly, working the crowd, caught in a circle of white light shining on his smile, his dark blue suit, white shirt, and red tie, as he bear-hugs, back-slaps , and handshakes his way up to the head table.

After two hours of creole chicken, boiled potatoes, and parboiled speeches, it is nearing 10:45 when Mondale finally rises to talk.

He eases into his keynote speech with a little low-key humor, including the true story he loves to tell about the vice-presidency:

''During that Three Mile Island disaster, where that power plant almost went off, they interviewed a woman who lived down the street from the plant who was not moving.

''They said, 'Madam, why aren't you moving away from this plant?'

''She said, ' 'Cause it's not dangerous.'

''They said, 'Why did you say it's not dangerous?'

''She said, 'Well, the President visited here yesterday.'

''They said, 'Why does that make it safe?'

''She said, 'Well, if it were dangerous, they would have sent the Vice-President.' ''

It breaks up the audience.

''And I say that,'' he continues smoothly, ''because I was just at the NAACP convention in New Orleans, and they sent that poor George Bush down there to explain why Reagan was good on civil rights. That's about as dangerous an undertaking as anybody could get.'' (Mr. Bush was booed by the convention.)

As the laughter subsides, without missing a beat, he adds:

''Will Rogers once said of (Herbert) Hoover, 'It's not what he doesn't know that bothers me, it's what he knows for sure that just ain't so. And that's the basic problem with the Reagan administration. Most of what they know for sure just ain't so.

''And we need a president who knows for sure what's going on . . . and leads this country again in a strong, solid course that puts people back to work, educates our children, enforces the laws that protect justice, extends liberties to protect women against discrimination through the Equal Rights Amendment and other ways, that reaches out to make certain that those who retire can retire with security, that stands up for the rights of organized labor in this country.

''You know, I believe in organized labor. Not just in Poland; I believe in it in the United States, too.''

And then comes the line that makes them roar like the ''Missou'' Tiger on the billboard coming into town: ''The government of the US is not up for sale. It belongs to the people of this country, and we intend to take it back!''

It is frankly partisan politics, preaching to the converted by the man who wants their votes as Democratic nominee.

He finishes the speech shortly after 11 and dashes for the cars, with two-thirds of the program yet to go. The Mondale caravan zooms through the black Missouri night to Columbia airport, where two silver charter planes wait, engines running. If it looks like the final scene from ''Casablanca,'' most of the group is too tired to notice.

Since the sun rose that morning over Clinton, Iowa, he has been in five cities for breakfast, receptions, a barbecue, and press conferences. The midnight flight to St. Louis is the midpoint of a grueling trip, with similar stops in Detroit; Little Rock and Jonesboro, Ark.; New Orleans; Milwaukee; Chicago; Cleveland; and Myrtle Beach, S.C.

In his barnstorming, Mondale hammers home his themes: Reduce the federal deficit, push for sustainable economic growth, repeal tax indexation and tax cuts for the wealthy, promote more private and public-service jobs, pass the Equal Rights Amendment, fund an $11 billion federal education plan with local emphasis, stress peacemongering rather than warmongering, cut the defense budget to realistic levels, put through a nuclear freeze, restore budget cuts in programs for the disadvantaged, end ''the feminization of poverty,'' and ensure the long-term integrity of social security.

He invariably emphasizes his contention that Ronald Reagan ''is not a conservative; he's a radical in every area of American life from economics to civil rights.''

Faced at one press conference with the recurrent criticism that he is a captive of special-interest groups (like labor and education), Mondale answers with relish: ''We have an administration that's working for every moneyed special interest in America. But someone like myself, who is standing up for medicare, medicaid, putting the unemployed back to work, helping handicapped citizens, enforcing civil rights legislation - that's a special interest? If so, count me in.''

The 16-hour campaign day that began Saturday in Clinton ended in St. Louis at 12:30 Sunday morning. Just after 9 a.m. Sunday the Mondale motorcade headed through the St. Louis heat wave, in air tasting like hot chrome, toward a political breakfast at Stan Musial & Biggie's Restaurant.

Mondale looked rested and dapper in a blue pin-stripe suit as he worked the room, moving counterclockwise around it toward the mike. He seemed to be carrying a suitcase full of small talk as he spotted familiar faces, chatted briefly, joked, moved on a few seconds later to the next outstretched hand.

Sen. Thomas Eagleton of Missouri, who was shortly to introduce him, called his friend of 23 years ''warm, compassionate, decent, ummmm . . . uninhibited, and concerned.'' Between bites of Danish pastry, Senator Eagleton said he sees the nomination as a two-man race between Mondale and Sen. John Glenn of Ohio, with Mondale the winner because of better political organization and instincts.

When he introduced Mondale, he emphasized the candidate's experience: 12 years as a senator and four years as vice-president, prepping for the presidency. Mondale picked up the cue:

''We need a president who understands the world in which we live and who reflects the values of our country in foreign affairs. Ignorance is a very dangerous foreign policy. It's not enough to scare the world with our arms. We must attract the world with our values and our leadership.''

Sounding a little like his mentor, the late Hubert Humphrey, he told the diners that he's a politician like them and proud of it, that ''politics is the business of freedom.''

Then the man whose voice had sometimes rasped during the Columbia speech told the sunlit St. Louis crowd, ''I appreciate your coming here tonight.''

It could have been a slip of the tongue - or an index of the brutal toll of night-and-day campaigning. But in a few minutes he and Joan Mondale, a lively brunette in a red blouse and brown skirt, moved off toward a press conference, where the mayor of St. Louis endorsed him.

Mrs. Mondale remained quietly in the background. Her husband, she said, is ''a peaceful man. He doesn't shout. He treats people with dignity. As president, I think he would be a healing, unifying influence.''

Asked to clarify that, she explained: ''I think Ronald Reagan is dividing the country - the rich from the poor, the young from the old, the employed from the unemployed.''

There is no concern, she thinks, for people having a difficult time. She said her husband is ''open, caring, concerned,'' and ''would have a 'let's do something' attitude.''

Mondale was born in Ceylon, Minn., the son of a Methodist minister and a music teacher. He grew up dirt poor and worked his way through Macalester College and the University of Minnesota, where he received a law degree.

The amazing series of political appointments that catapulted him to high office began when Mondale, as a 32-year-old lawyer, was named Minnesota attorney general, then US senator to fill out Vice-President Humphrey's term, and finally vice-presidential nominee by Jimmy Carter.

But once in the Senate, Mondale won reelection twice on his own, with a record that focused on defending the defenseless in society: the poor, hungry, unemployed, migrant workers, children, and families under siege.

In an interview in his Washington office, Mondale said he didn't consider being a liberal a handicap. As president, he said, he would require a family-impact statement on all legislation as well as set up a council of social advisers similar to the Council of Economic Advisers.

''We consider lots of things on committees, but not the effect produced by some cuts on the American family, which is the basic source of the nation's strength and ethical training,'' he said. ''Families are breaking up. There's more wife abuse, child abuse, drug addiction, even suicides.''

Mondale himself is one of six children. He has been married for 27 years to the former Joan Adams, the daughter of a Presbyterian minister. They have three children: Ted, Eleanor Jane, and William.

In Washington, the Mondales live in a comfortable but not flossy house of white stucco, with a porch. When Mondale left the Senate he was worth about $77, 000. Now a counsel to the Chicago-based law firm of Winston & Strawn, he has earned about $700,000 since leaving office, enough to have recently built a second home in Minnesota designed by a disciple of Frank Lloyd Wright.

His office at Winston & Strawn in Washington looks surprisingly as his Senate office did - spare. There are color photos of Minnesota lakes and woods on the vanilla-hued walls, but none of the Cezanne or other prints that adorn even the photocopy room at Winston & Strawn.

He strides across the room, offers a warm handshake, sprawls in a cream- and tan-striped chair under a Hubert Humphrey plaque bearing the words ''A fellow that doesn't have any tears doesn't have any heart.''

Fritz Mondale, once critized as a plodding speaker, now can stir an audience into a political foam, as he did recently when the National Council of Senior Citizens convention in Washington gave him a standing ovation.

But off the campaign trail he is almost anticharismatic. He is both warm and distant. He makes no attempt to charm, disarm, or win approval.

With Mondale, says Maxine Isaacs, ''what you see is pretty much what you get.'' She has worked for him for more than 10 years and is now his press secretary.

What you see is a man of medium height whose once dark blond hair is gray, streaked with silver, a man with a ruddy complexion, strong features dominated by an ice-cutter of a nose that gives him a Barrymore profile, and eyes as deep blue as a Norwegian fjord. He is handsomer in person than on TV, where he sometimes looks lugubrious and uncomfortable.

In his book ''The Accountability of Power,'' Mondale devotes a chapter to ''The Presidential Personality and Public Leadership'' and notes: ''Americans want to know whether politicians have a healthy perspective on the world and on themselves; whether they have bloated egos capable of further distortion, whether they will remember that people put them in office and that they bear a responsibility to them.''

Those who know Mondale point to his stability, decency, and to the fact that, as Humphrey said, he does not polarize people.

But his biographer, Finlay Lewis, in ''Mondale: Portrait of an American Politician,'' questions whether ''he lacked the ultimate lust for power'' to be president.

Mr. Lewis cites three instances in which Mondale dropped out of races, the most significant his surprise announcement in 1974 that he had decided not to be a candidate for the presidency after 14 months of preparing for it.

Mondale, however, says: ''All the fights I've been involved in, I saw them all through. Part of leadership is knowing when you're ready. I really wasn't ready in '74. I knew it, and I got out. Now I am ready.''

Former US Sen. John Culver of Iowa sees Mondale's decision to step out of the race in '74 as a healthy sign. ''He's not driven in a way that's unhealthy and suspect. Politics at all levels tends to become a need for fulfillment. A look at history shows us some [leaders] who are examples of instability, driven by insecurity, the need to sacrifice for public acclaim and approval as a certification of worth.''

Mondale's longtime friend and campaign manager, James Johnson, rebuts the question Humphrey once raised about whether Mondale had the ''fire in the belly'' to charge through a long presidential race.

''Look at his actions,'' Johnson says. ''He's been doing everything possible to win. As he says, he's been on the road more than any other living American. He's worked night and day. Last year he was in 42 states. He's been on the road 150 days this year, done 225 state hits, has been in 40 states already this year.''

In the first six months of the year Mondale raised $5.1 million for his war chest, says Johnson, noting that his closest competitor, Glenn, has raised only

The Mondale campaign, with its staff of 79, is recognized inside the party as the best organized of all the candidates' and is sweeping up endorsements the way a magnet picks up pins.

''We have a substantial number of senators, congressmen, mayors, and governors,'' Johnson admits modestly.

Mondale, the minister's son, says: ''I often pray to have the courage and wisdom to govern wisely. The public trust is everything. Government without it is nothing, and the public trust has been shattered so many times.'' He refuses to speculate, however, when asked what happened with the Carter briefing book. Let someone else fish in those troubled waters is his attitude.

Fishing, in fact, is the ideal sport and perhaps the ideal political metaphor for this man who is described as gregarious with people but reticent about himself.

''Fishing is a very complex, mysterious sport,'' he says. ''You get out on a lake you've never been on before; where are the fish? What do they like and how deep are they? What species are they? Are they biting, heavy or light, or are they biting at all? Should you use live bait or a lure? You have to figure out what they're doing. It is a high and mysterious art form known to few.''

He says it softly, eyes half closed, a faint smile on his face - this man who has gone fishing for the presidency.